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Aviation History
1933
1933 - 1280.PDF
FLIGHT, DECEMBER 21, 1933 Fig. 15 : Representative types of salt-cooled valves. For the crankshaft, the engine designer has the various alternatives of case-hardening alloy steel, nitrided steel of the nitralloy type, or nitriding chrome molybdenum steel. The Brinell hardness obtainable with the first and third types is about 650 to 750, while nitralloy offers a hardness of 800 to 1,100. I believe that chrome molybdenum steel offers the best solution, owing to the freedom from difficulties experienced when case-hardening large diameter shafts. For big end bearings, the most promising alternatives I suggest are steel-backed lead bronze, or the needle roller type. It would appear that the former is the most hope ful immediate solution, because the latter, although having many attractive points, presents difficult problems—espe cially in the larger sizes. (iv) Reduction Gear. Increased rotational speed necessitates the adoption of airscrew reduc tion gear, if propeller efficiencies are to be maintained ; and the use of reduction gears on high-powered engines has been general practice in England for the last few years. The Bristol Company, on their range of radial air-cooled engines, have not produced any ungeared engines for the past three years. From the type of future engine outlined in this paper, the necessary power will be obtained by a combination of high brake mean effective pressure and high revolutions per minute, which will necessitate reduc tion gears being universally adopted, even on moderate power and smaller size engines. Mr. B. Waseige, Technical Director of Messrs. Farman, France, has recently laid down that an efficient two-bladed airscrew should pass 3.35 lb. of air per horse-power/ second, whilst tip speeds ought not to exceed 820 ft. per sec. for low altitude machines and 980 ft. per sec. for high altitude aircraft. Concentric reduction gears are desirable for air-cooled radial engines to obviate air flow interferences, and the Farman bevel reduction gear, as standardised on Wright and Bristol engines, offers several advantages. The Far- man type is very robust, gives considerable damping effect to the crankshaft system, and enables a wide range of propeller speed reduction to be obtained. With the in crease of crankshaft rotational speed, reduction ratios of more than 2 to 1 will be necessary. Fig. 16 shows a selection of Bristol reduction gears for radial air-cooled engines. Lubrication Since the introduction, by the British Air Ministry, of a straight mineral oil specification some three years ago, lubrication problems in England and America have been generally of a similar nature, except that I understand higher viscosity oils are used on air-cooled engines in America than are permitted in England. There are two points I would like to touch upon, namely, oil consumption and sludging. In an interesting paper on air-cooled aero engine design, read before the S.A.E. in America, by Mr. Philip Taylor, in April, 1931, an instructive curve was shown illustrating the consumption on air-cooled engines as being extremely critical, and suggesting that a rapid rise is reached after a certain engine speed. As already stated, I expect to see a considerable increase in revolutions per minute on future types of air-cooled engines, and this question of high oil consumption, from the point of view of the total weight of the power plant for a given range, is of the utmost im portance to aircraft designers. Reduction in oil consump tion on the air-cooled radial engine is susceptible to treat ment along several lines, such as: From the cylinder aspect, stiffer cylinder flanges, barrels and skirts ; from the piston aspect, by suitable detail design modifications ; and from the crankcase aspect by means of more efficient breathing and scavenging of the oil. Oil temperature is also an important factor affecting oil consumption and sludging, and I suggest that we may hope that the oil technologists will, within a reasonable time, provide lubricating oils which may be employed with Fig. 16 : Bristol reduction-gear units of different ratios. On right, Fig. 17, a nose-type oil cooler. 1290
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